(i) Field of the Invention
The present invention relates to a vacuum adiabatic type coolant container for holding low temperature coolants such as liquid nitrogen, liquid helium, or liquid hydrogen.
(ii) Description of Related Art
Japanese Patent Laid-Open Publication No. 57-195998 describes a conventional coolant container made of fiber reinforced plastics (hereinafter referred to as "FRP"). According to this document, the coolant container is made by joining together the cylinder which forms the inner surface of the container and the panel which forms the bottom surface. It also suggests the possibility of forming these as a single unit.
FIG. 4 is a cross section of an example of a conventional coolant container wherein the inner container comprising a cylinder and a panel are built inside the outer container formed as a single unit. A coolant container 30 in a substantially cylindrical shape is comprised of a cup-shaped, single-unit outer container 31 and an inner container 32 placed inside the outer container 31. The inner container 32 is comprised of a panel 322 in a simple disk shape that engages with the lower end of a round cylinder 321 (the lower end as shown in FIG. 4; hereinafter "lower" and "upper" are used in relation to the figures). There is a flange on the upper end of the inner container 32 which fits the open end of the outer container 31, and the space enclosed by the outer container 31 and the inner container 32 is maintained with a vacuum and forms a vacuum adiabatic portion 13.
When a coolant 6 is held inside the inner container 32 of coolant container 30 constructed in this way, evaporation of the coolant 6 is restricted by the adiabatic effect of the vacuum adiabatic portion 13, and as a result the coolant 6 can be maintained for a long period of time.
FIG. 5 is a cross section of an example of another conventional coolant container and shows the construction of the container disclosed in the above Laid-Open Patent. This coolant container 40 comprises a cylinder 321 in a simple, rounded-edge, cylindrical shape engaged with the inner surface of the upper end of a panel 402 formed in the shape of a short cup and bonded together.
However, the following problems arise with the conventional coolant container 30.
1) When external pressure such as or approximately the same pressure with an atmospheric pressure acts on the upper surface of the panel 322 and the inner surface of the cylinder 321 of the inner container 32 shown in FIG. 4 from toward the bottom), there is a danger that with the engagement configuration of the inner container 32 the panel 322 will sink down against the lower side of the vacuum adiabatic portion 13 and the coolant maintenance capability of the container will decline. If the vacuum is broken in this way, the adiabatic capability of the vacuum adiabatic portion 13 will be lost and the coolant can easily evaporate due to heat transfer from the outside. It is therefore necessary to reinforce the container by processing a internal thread on the inner periphery of the lower end of the round cylinder 321 and a external thread that fits the internal thread on the panel 322, and fitting the external thread into the internal thread to prevent the panel 322 from sinking down. The same reinforcement as the inner container 32 will be necessary for the outer container 31 if it, like the inner container 32, is cylindrical shape with an open lower end closed off with a disk-shaped panel, because external pressure such as approximately the same pressure with atmospheric pressure will act from below on this panel. Construction of the screw and nut is troublesome and because it is necessary to increase the thickness of the panel 322 in order to obtain a designated screw-mesh force, the cost of materials mounts.
Even when the inner container 40 of the engagement configuration shown in FIG. 5 is used as the inner container 32 in the coolant container 30, the danger that the panel 402 will sink down toward the vacuum adiabatic portion 13 side of the lower end and the capability of the container to maintain the coolant will decline cannot be ruled out.
2) If the cup-shaped outer container 31 as shown in FIG. 4 is made of FRP for example, a mold is produced and the FRP layers are layered by hand one at a time on top of the mold while coating with a matrix resin to form a single-unit container (hand lay-up method). However, this manufacturing method is time-consuming and manufacturing costs are high. A method in which the FRP and resin are sprayed together onto the mold (spray-up method) can be applied, but again the manufacturing costs are high and there are limits to the shape of the mold.